01 - Cream softened butter with brown sugar until fluffy in a small bowl. Whisk in milk and vanilla. Fold in heat-treated flour and salt until just combined. Gently mix in mini chocolate chips. Form 12 balls (about 2 tsp each) and arrange on parchment-lined plate. Refrigerate until firm.
02 - Combine butter and chopped dark chocolate in a heatproof bowl. Microwave in 30-second intervals, stirring between each, or melt over a double boiler. Stir until completely smooth and set aside to cool slightly.
03 - Beat granulated sugar, brown sugar, and eggs in a large bowl for 2 minutes until pale and thickened. Whisk in vanilla extract followed by the cooled melted chocolate mixture until fully incorporated.
04 - Whisk flour, cocoa powder, baking powder, and salt in a separate bowl. Add dry ingredients to wet mixture, folding until a thick, fudgy dough forms. Refrigerate for 10-15 minutes for easier handling.
05 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
06 - Scoop approximately 2 tablespoons of brownie dough per cookie. Flatten in palm, place one chilled cookie dough ball in center, and wrap brownie dough completely around filling, sealing edges. Repeat with remaining dough.
07 - Arrange cookies on prepared baking sheets, leaving space between each. Bake 10-12 minutes until tops are set with characteristic crackled surface. Avoid overbaking for ideal fudgy texture.
08 - Let cookies rest on baking sheets for 10 minutes to set before transferring to wire rack to cool completely.